Low carbon welded tube and process of manufacture thereof

ABSTRACT

The present invention relates to low carbon welded tubes and process of manufacture thereof. Manufacture of welded tubes utilising draw bench processes is energy intensive process due to the need of multiple pass (to enhance mechanical properties, in particular tensile and yield strength) to reduce the diameter of the hollow to desired dimensions as only 20 to 35% reduction achievable in a single pass. Further, there is substantial material loss with every pass and poor control on dimensional stability and surface finish of the tube. The present invention provides a synergistic combination of induction and/or resistance heat treatment of the hollow and the cold rolling process resulting in remarkable reduction in energy consumption, and at the same time enhancing quality of the resulting tubes with improved dimensional stability, closeness of tolerance, reduced thickness variation, concentricity and substantial reduction in material wastage.

FIELD OF INVENTION

The present invention relates to low carbon welded tubes and process of manufacture thereof. In particular, the invention relates to cold rolled low carbon welded tubes, system and process of manufacturing of the same.

BACKGROUND OF THE INVENTION

Tubes are used in diverse applications involving automobile, boilers, textiles, construction, scaffolding, energy sector, hydraulic cylinders, gas springs etc., that are either manufactured from carbon steel or alloying elements. Tubes with carbon percentage of 0.01 to 0.45, are conventionally known as low carbon steel tubes. Seamless tubes are manufactured by the extruding the stock whereas welded tubes are manufactured from formed strip that is welded at the seam. Welded tubes are used in applications requiring stringent dimensional tolerances, surface finish and mechanical properties such as yield strength and tensile strength.

The welded tube manufacturing processes generally comprise steps that include

-   -   strip cutting according to the final tube size;     -   forming of the strip;     -   Electric Resistance Welding (ERW) welding along the seam of the         formed strip to produce hollow;     -   Heat treatment to relieve stresses;     -   Surface treatment;     -   pointing operation wherein part of the tube is squeezed to         provide holding margin/region for the draw bench gripper which         is used to pull the tube through a draw bench in which the         squeezed portion which cannot be used becomes process waste;     -   drawing process wherein the hollow is drawn through a draw bench         to reduce the tube diameter to a desired level;     -   straightening;     -   heat treatment to relieve stresses (optional);

The reduction of tube diameter and thickness from the hollow (formed tube in welded condition) is necessary to achieve the desired dimensions and in enhancing mechanical properties such as yield strength, tensile strength, percentage elongation and hardness of the tube. The tensile and yield strength is proportional to the percentage reduction of the hollow to final tube diameter and thickness. Using a draw bench, the reduction of the tube diameter and thickness is limited to only 35% in one pass.

Generally the hollow used is of cross section/diameter that is 40% to 50% more than that of the final drawn tube thereby requiring multiple passes through the draw bench to achieve desired dimensions and mechanical properties. For each pass through draw bench, it is necessary to heat treat the tube and provide pointing region which is of the order of 7% of the weight of the tube. This results in a severe material loss of about 7% with substantial energy consumption during the heat treatment. Such a process also requires repeated tube straightening and surface treatment resulting in lower dimensional stability and tolerances.

Seamless tubes are preferably used in critical application in preference to welded tubes which are susceptible to cracking, weld opening and failure under pressure.

PRIOR ART

United States Patent Application 20050076975 discloses a low carbon alloy steel tube and a method of manufacturing the same, in which the steel tube consists essentially of, by weight: about 0.06% to about 0.18% carbon; about 0.5% to about 1.5% manganese; about 0.1% to about 0.5% silicon; up to about 0.015% sulfur; up to about 0.025% phosphorous; up to about 0.50% nickel; about 0.1% to about 1.0% chromium; about 0.1% to about 1.0% molybdenum; about 0.01% to about 0.10% vanadium; about 0.01% to about 0.10% titanium; about 0.05% to about 0.35% copper; about 0.010% to about 0.050% aluminum; up to about 0.05% niobium; up to about 0.15% residual elements; and the balance iron and incidental impurities. The steel has a tensile strength of at least about 145 ksi and exhibits ductile behavior at temperatures as low as −60° C.

Japanese Patent JP3077576 discloses the welded tube manufactured by forming a strip steel containing ≦0.05% C and 10 to 14% Cr, by weight, into a pipe shape, subjecting both butted edge parts whose temperature is between the room temperature and 1000° C., to laser beam welding by the following conditions (1) and (2), and are heated in between 850 and 1000° C., cooling them to ≦300° C. at ≧20° C./s, heating them to 600 to 700° C., then cooling them to the room temperature at ≦20° C./s. (1) P≧15 kW (2) 0.4≦P{exp (a.T)}/(V.t)≦2, (where, P: laser beam output (kW), a: constant (=0.0006), T: temperature before welding (° C.), V: welding speed (m/min), t: Thickness of strip steel (mm)). Also the welded tube is manufactured by heating the above-mentioned welded tube to 700-900° C., then cooling it to the room temperature at ≦20° C./s.

Japanese Patent JP09164425 discloses the Welded tube manufactured by forming a strip steel containing ≦0.05% C and 10 to 14% Cr, by weight, into a pipe shape, subjecting both butted edge parts whose temperature is between the room temperature and 1000° C., to laser beam welding by the following conditions (1) and (2), and are heated in between 850 and 1000° C., to cooling them to ≦300° C. at ≧20° C./s, heating them to 600 to 700° C., then cooling them to the room temperature at ≦20° C./s. (1) P≧15 kW (2) 0.4≦P{exp (a.T)}/(V.t)≦2, (where, P: laser beam output (kW), a: constant (=0.0006), T: temperature before welding (° C.), V: welding speed (m/min), t: Thickness of strip steel (mm)). Also the welded tube is manufactured by heating the above-mentioned welded tube to 700-900° C., then cooling it to the room temperature at ≦20° C./s.

Japanese Patent JP11254030 discloses a stainless steel strip incorporating, by weight, 12.0-15.0% Cr, 1.0-5.0% Ni, ≦0.030% C+N is worked into a tubular shape, and both butted edge parts are welded with laser beams, next heated at 730-900° C. for 2-60 sec, next cooled down to ≦150° C., next heated at 580-770° C. for 1-30 sec and next cooled down to an ordinary temperature.

Japanese Patent JP11343519 discloses a hot rolled steel plate, which has a composition, by weight, consisting of ≦0.05% C, ≦1.0% Si, ≦5.0% Mn, ≦0.04% P, ≦0.01% S, 10.0-15.0% Cr, 0.1-3.0% Mo, ≦0.1% Al, ≦0.10% Ti, Ni satisfying the formula: 3.0−0.5×Mn≦Ni≦8.0−0.5×Mn and the balance Fe with inevitable impurities, is annealed. The hot rolled steel plate softened is formed into a tubular shape and butted parts are welded to obtain a tube. The welded tube is held at 850-1250° C. for about ≧10 min and then is subjected to a post heat treatment. At this time, the post heat treatment satisfies conditions shown in the formula: 2000×Mo+T2(20+log t2)≧T1 (20+log t1) wherein T1 and t1 are a temp. and a time of annealing, T2, t2 are a temp. and a time of post heat treatment.

Japanese Patent JP2000126896 discloses the method for manufacturing the low carbon martensitic stainless steel welded tube by which a low carbon martensitic stainless steel strip is continuously formed into a tubular shape with plural roll-forming stands, and both edge parts to be butted of the formed tubular steel is heated and subjected to laser beam welding, fin pass rolls for holding the interval between both these edge parts at a prescribed distance and squeeze side rolls for pressurizing and butting both these edge parts are provided and also a lifting-roll device 7 is arranged between the final fin pass roll 3a and the squeeze side rolls 6. The gap G in the height direction of both edges is measured and butt welding is executed while adjusting the amount of lift with the lifting-roll device 7 based on the measured results.

EP0217751 discloses a process is described for manufacturing steel tubes and steel pipes by electric welding of strips so formed to get a complete hollow bar, wherein the steel strip is pre-heated before forming. The pre-heating temperature is preferably near to the welding temperature and possibly, at the exit from a furnace which may be electric, e.g. of the induction type, a gas furnace or an oil furnace, etc. and before the forming unit there can be provided means capable of accomplishing an edge conditioner step.

Japanese Patent JP10128413 discloses three mandrels 2 are respectively arranged in the insides of three tube stocks 1 which are arranged in parallel, three pairs of grooved rolls 5 are coaxially connected, each pair of grooved rolls is arranged on the peripheral surface of each tube stock so that the inside face of the groove 6 of the roll is brought into contact with the outer peripheral surface of each tube stock and three tubes are simultaneously rolled. The mandrel 2 has a rolling part 3 whose diameter is gradually decreased in the rolling direction and the distance between the groove bottom and the center axis 7 of the roll of the grooved roll 5 is continuously changed in accordance with the change of the diameter of the rolling part. The groove shape and dimensions of the three sets of the grooved rolls 5 and mandrels 2 are set so that the distributions of their working ratios are mutually substantially same and, by the one set of combination, the rolled tube having the diameter dimension different from that of the rolled tubes which are rolled with the other sets of combinations is obtained.

Japanese Patent JP58144455 discloses a roll material for Pilger rolling comprising 1.5W2.5% C, 0.2W 1.2% Si, 0.2W1.2% Mn, 0.5W2.0% Cr, 4W8% V and the remainder Fe and inevitable impure elements, capable of obtaining necessary hardness in the surface layer part thereof by proper heat treatment, rich in internal toughness, excellent in anti-wear property and having good grindability and long life is obtained. In the above mentioned composition, C permits to precipitate a large amount of carbide of V to impart anti-wear property to the roll material and strengthen the matrix of steel. V is contained in a degree generating no macro-segregation of V, and Cr is contained in a degree capable of affording proper tempering property.

Japanese Patent JP2005060796 discloses the welded steel tube having the composition composed by mass % of 0.02-0.2% C, ≦1% Si, 1.5-4% Mn, ≦0.1% P, ≦0.01% S, ≦0.1% Al, ≦0.01% N, ≦0.1% Ti, ≦0.1% Nb, ≦0.01% B, a reducing-rolling at ≧700° C. rolling-finishing temperature and ≦35% accumulated shrinkage diameter ratio is applied and the obtained steel tube is used as a blank steel tube, and a cold-drawing treatment is applied to this blank steel tube to form the steel tube having a prescribed size. Then, after cold-drawing treatment, an annealing treatment can be applied. Further, one or more kinds among Cu, Ni, Cr, Mo and/or one or two kinds of Ca and REM can be contained.

Japanese Patent JP3485980 discloses a clad steel tube is manufactured by achieving the cladding by welding of the corrosion resistant or heat resistant Ni—Cr—Mo alloy with a steel tube made of carbon steel, alloy steel, stainless steel, heat resistant steel or the like as the stock tube. The cold working or warm working of rolling, drawing, etc., of the clad steel tube is achieved, and the heat treatment is further achieved at the recrystallization temperature or at a higher temperature. Heating is made for the prescribed period of time at the temperature of ≧1100° C. as the heat treatment of the solid solution of the Ni—Cr—Mo alloy at the outer circumferential part of the clad steel tube to realize the recrystallization. The heat treatment is achieved according to the material at the inner circumferential part of the clad steel tube to realize the recrystallization. Rolling and drawing is achieved not in the hot condition, but in the cold or warm condition because the high temperature strength of the base metal is different from that of the layer cladded by welding, and the uniform working is not achieved in the hot condition and flaws are generated.

Japanese Patent JP2001303196 discloses a hot rolled or cold rolled hoop stock, which has a composition consisting of 0.01-<0.05% C, ≦1.0% Si, ≦3.0% Mn, ≦0.15% P, ≦0.015% S, ≦0.04% Al, 0.005-0.02% (and ≧0.003% in a state of solid solution) of N and the balance Fe with inevitable impurities and containing, if necessary, at least one kind selected from 0.005-0.040% Nb, 0.005-0.50% Ti, 0.005-0.020% B, 0.02-1.5% Cu, 0.02-1.0% Ni, 0.02-1.0% Cr, 0.02-1.0% Mo, 0.0020-0.02% Ca and 0.0020-0.02% REM, is formed into cylindrical shape and the resultant seam is subjected to electric resistance welding, followed by sizing at 0.3-10% drawing rate of outer peripheral length.

Japanese Patent JP2001303195 discloses a hot rolled or cold rolled hoop stock, which has a composition consisting of 0.01-<0.05% C, ≦1.0% Si, ≦1.0% Mn, ≦0.15% P, ≦0.015% S, 0.01-0.1% Al and the balance Fe with inevitable impurities and containing, if necessary, at least one kind selected from 0.005-0.040% Nb, 0.005-<0.05% Ti, 0.0005-0.020% B, 0.02-0.5% Cu, 0.02-1.0% Ni, 0.02-1.0% Cr, 0.02-1.0% Mo, 0.0020-0.02% Ca and 0.0020-0.02% REM, is formed into cylindrical shape and the resultant seam is subjected to electric resistance welding, followed by sizing at 0.3-10% drawing rate of outer peripheral length.

Japanese Patent JP2001303192 discloses a hot rolled or cold rolled hoop stock, having a composition which consists of, by mass, 0.001-<0.01% C, ≦1.0% Si, ≦52.0% Mn, ≦0.15% P, ≦0.015% S, 0.01-0.10% Al, 0.01-0.10% Nb, 0.001-0.010% B, either or both of ≦0.10% Ti and ≦0.10% Zr, and the balance Fe with inevitable impurities and contains, if necessary, either or both of 0.002-0.5% Mo and 0.02-1.0% Cr and in which C, Nb, Ti and Zr are contained in amounts within the range satisfying (12/48)(Ti (%)/C (%))+(12/93)(Nb (%)/C (%))+(12/91)(Zr (%)/C (%)≧1.0, is formed into cylindrical shape and the resultant seam is subjected to electric resistance welding, followed by sizing at 0.3-10% drawing rate of outer peripheral length.

Japanese Patent JP2618563 discloses a slab of a material steel which has a composition consisting of, by weight, 0.10-0.20% C, 0.15-0.50% Si, 1.3-2.5% Mn, 0.005-0.020% P, 0.0005-0.0060% S, 0.01-0.08% Al, 0.02-0.2% Ti, 0.0010-0.0030% B, 0.002-0.005% N, 0.3-0.7% Cr, 0.3-1.0% Mo, and the balance Fe with inevitable impurities and further containing, if necessary, 0.01-0.10% Nb is hot-rolled. The finishing temp. is regulated to a value between 950° C. and the Ar transformation point, and coiling is 3 done at 450-700° C. The resulting hot rolled coil is formed into a tube by means of resistance welding and then subjected, if necessary, to normalizing, annealing, and cold drawing. By this method, the resistance welded steel tube having a dimensional accuracy of ≦±0.15 mm outside diameter and ≦±0.05 mm thickness and also having (100 to 130) kgf/mm2 tensile strength can be obtained

Japanese Patent JP08103867 discloses a clad steel tube is manufactured by achieving the cladding by welding of the corrosion resistant or heat resistant Ni—Cr—Mo alloy with a Steel tube made of carbon steel, alloy steel, stainless steel, heat resistant steel or the like as the stock tube. The cold working or warm working of rolling, drawing, etc., of the clad steel tube is achieved, and the heat treatment is further achieved at the recrystallization temperature or at a higher temperature. Heating is made for the prescribed period of time at the temperature of ≧1100° C. as the heat treatment of the solid solution of the Ni—Cr—Mo alloy at the outer circumferential part of the clad steel tube to realize the recrystallization. The heat treatment is achieved according to the material at the inner circumferential part of the clad steel tube to realize the recrystallization. Rolling and drawing is achieved not in the hot condition, but in the cold or warm condition because the high temperature strength of the base metal is different from that of the layer cladded by welding, and the uniform working is not achieved in the hot condition and flaws are generated

Japanese Patent JP06010046 discloses a slab of a material steel which has a composition consisting of, by weight, 0.10-0.20% C, 0.15-0.50% Si, 1.3-2.5% Mn, 0.005-0.020% P, 0.0005-0.0060% S, 0.01-0.08% Al, 0.02-0.2% Ti, 0.0010-0.0030% B, 0.002-0.005% N, 0.3-0.7% Cr, 0.3-1.0% Mo, and the balance Fe with inevitable impurities and further containing, if necessary, 0.01-0.10% Nb is hot-rolled. The finishing temp. is regulated to a value between 950° C. and the Ar transformation point, and coiling is 3 done at 450-700° C. The resulting hot rolled coil is formed into a tube by means of resistance welding and then subjected, if necessary, to normalizing, annealing, and cold drawing. By this method, the resistance welded steel tube having a dimensional accuracy of ≦±0.15 mm outside diameter and ≦±0.05 mm thickness and also having (100 to 130) kgf/mm2 tensile strength can be obtained

Japanese Patent JP05287371 discloses in a resistance welded steel tube having a componental compsn. obtd. by incorporating., by weight, 0.15 to 0.40% C, 0.05 to 0.50% Si, 2.0 to 3.0% Mn, 0.005 to 0.020% P, 0.0005 to 0.0060% S, 0.01 to 0.08% Al, 0.01 to 0.20%, Ti, 0.001 to 0.003% B, 0.002 to 0.0050% N, 0.1 to 1.0% Mo and 0.1 to 0.3% V with one or more kinds of 0.1 to 0.7% Cr and 0.01 to 0.20% Nb, and the balance Fe with inevitable impurities, after the tube making, normalizing is executed as heat treatment. If required, normalizing is furthermore executed in the process of cold drawing and after the cold drawing. In this way, the objective resistance welded steel tube having ≧150 kgf/mm2 tensile strength and ≧10% elongation can be obtd

Japanese Patent JP04365815 discloses a steel having a composition consisting of by weight, ≦0.01% C, ≦0.05% Si, ≦0.30% Mn, ≦0.025% P, ≦0.015% S, ≦0.080% sol.Al, 0.002-0.10% Ti and/or Nb, and the balance Fe with inevitable impurities is hot-rolled to the prescribed plate thickness at ≧(Ar3+40° C.) hot strip finishing temp. and at ≦500° C. coiling temp. The resulting hot rolled steel plate is cooled, formed into tubular state, and subjected to electric resistance welding. The resulting steel tube is heat-treated at 700-900° C. and finished by means of cold reduction. By this method, the number of drawing times can be reduced by the increase in reduction of area per time at drawing and manufacturing costs can be remarkably reduced. Further, the extention of use can be expected by the increase in workability

Japanese Patent JP01108346 discloses the steel for electric welded steel tube has a composition consisting of, by weight, 0.003W0.20% C, ≦1.0% Si, 0.1W0.8% Mn, ≦0.03% P, ≦0.02% S, 0.005W0.025% SolAl, ≦0.0035% N, and the balance Fe with inevitable impurities and also has superior cold workability. By using the above steel, the electric welded steel tube in which the amount of AlN precipitation in an electroseamed zone is equal to that in a base metal part can be obtained, and further, by subjecting the as-weld electric welded steel tube to cold drawing, a cold-drawn steel tube having high cold formability can be obtained

Japanese Patent JP3030602 discloses at the time of producing a resistance welded steel tube by using a hot-rolled coil sheet 1 with ribs, the gap of break down rolls 3a of a forming line are made≦(stock thickness+rib height+2 mm) and ≦4 mm, and the reduction amount at a coil edge part at the time of resistance welding with fin pass rolls 4 is made ≧0.1×(stock thickness+rib height) and ≦0.5×(stock thickness+rib height). Further, the reduction amount at sizing rolls 7 is made ≧0.3%, ≦1.2% of the length of circumference of the outermost face of steel tube before reduction and then the resistance welded steel tube 10 is produced. Therefore, when forming, crushing of ribs and welding of mutual ribs are suppressed, the productivity is improved, the production cost is decreased, and further, the spectacle as the steel tube is improved.

Japanese Patent JP2006136927 discloses a cold pilger rolling process, in the preventing method of the end crack by heating an end part of the tube stock before rolling, the end crack of a rolled stock is prevented by heating the end part of the tube stock before the pilger rolling. In a rolling with a cold pilger mill, in the heating device of the end part of the tube stock before rolling, the end part of the tube stock is heated by discharging the tube stock before the pilger rolling from a tube stock table with a kicker, placing it on a free roller after the kicker is lowered in a fixed state to a kicker stopper, advancing it by the dead weight and bringing the end part of the tube stock into close contact with a heating nozzle.

European Patent EP 0217751 discloses process for manufacturing of electrically welded steel tubes and pipes from pre-heated steel strip wherein the strip is preheated before forming preferably near welding temperature.

Manufacture of welded tubes utilising draw bench processes suffer from limitations such as:

-   -   Need of multiple pass (to enhance mechanical properties, in         particular tensile and yield strength) to reduce the diameter of         the hollow to desired dimensions as only 20 to 35% reduction         achievable in a single pass     -   Heat treatment, pointing, surface treatment in every pass         resulting in high energy consumption without getting         commensurate benefits     -   Substantial material loss with every pass     -   poor control on dimensional stability and surface finish of the         tube

There is a need of providing energy efficient cold rolled processes for the production of low carbon welded tube with surface finish, closeness of tolerance and mechanical properties that are on par with the seamless tubes produced by fairly energy intensive processes.

SUMMARY OF THE INVENTION

The main object of the invention is to provide an energy efficient process for the manufacture of low carbon welded tubes.

Yet another object of the invention is to obviate multiple passes adapted during the conventional tube manufacturing processes.

Yet another object of the invention is to achieve dimensional tolerances, surface finish and mechanical properties such as yield strength and tensile strength of low carbon welded tubes.

Yet another object of the invention is to reduce cycle time for production of the welded tubes.

Another object of the invention is to provide heat treatment process for the welded tube.

Another object of the invention is to provide a system for cold rolling process.

Yet another object of the invention is to provide dies and mandrel for the cold rolling process.

Yet another object of the invention is to provide tube feeding mechanisms for cold rolling process.

Thus in accordance with the invention, the process for manufacture of the cold rolled welded tubes comprises steps of:

-   -   strip cutting as per the desired final tube size;     -   forming of a strip;     -   subjecting the strip to high frequency induction welding along         the seam of the said strip to produce hollow wherein the coil in         the proximity of the tube and welding rolls induces a magnetic         field that is concentrated on the open seam by an impeder which         is disposed inside the tube resulting in the generation of heat         along the seam to fusion temperature; followed by fusing of the         open seam to complete the welding process to produce hollow;     -   heat treating the hollow;     -   optionally surface treating the hollow;     -   cold rolling the hollow, wherein the hollow is displaced forward         in the direction of rolling under cam shaped profiled rolls         rotatably mounted in an oscillated roll stand wherein the hollow         is moved forward stepwise under the rolls, wherein the profiles         of the rolls are shaped so that on rotation they bite into the         hollow to forge it down onto the mandrel that is disposed inside         the hollow, simultaneously the said hollow with the mandrel is         rotated about its longitudinal axis as hollow advances between         the said rolls wherein the mandrel is then moved back to drag         the hollow against the rolls and the process is repeated for         next segment of the hollow.

DETAILED DESCRIPTION OF THE INVENTION

Features and advantages of the invention will become apparent in the following detailed description and preferred embodiments with reference to the accompanying drawings

FIG. 1 Process flow chart of the conventional process (Sheet 1)

FIG. 2 Process flow chart of the process of present invention (Sheet 1)

FIG. 3 Microstructure representation of drawn tube (Sheet 2)

FIG. 4 Microstructure representation of cold rolled tube (Sheet 3)

FIG. 5 Variation of Grain size with respect to Area fraction (Sheet 4)

EXPLANATION OF TERMS

Cold rolling: The process of cold rolling in this application refers to the process wherein hollow is displaced forward in the direction of rolling under cam shaped profiled rolls rotatably mounted in an oscillated roll stand wherein the hollow is moved forward stepwise under the rolls, wherein the profiles of the rolls are shaped so that on rotation they bite into the hollow to forge it down onto the mandrel that is disposed inside the hollow, simultaneously the said hollow with the mandrel is rotated about its longitudinal axis as the hollow advances between the said rolls wherein the mandrel is then moved back to drag the hollow against the rolls and the process is repeated for next segment of the hollow.

The process for manufacture of the cold rolled welded tubes comprises steps of:

-   -   strip cutting as per the desired final tube size;     -   forming of a strip;     -   subjecting the strip to high frequency induction welding along         the seam of the said strip to produce hollow wherein the coil in         the proximity of the tube and welding rolls induces a magnetic         field that is concentrated on the open seam by an impeder which         is disposed inside the tube resulting in the generation of heat         along the seam to fusion temperature; followed by fusing of the         open seam to complete the welding process to produce hollow;     -   heat treating the hollow at 650-950° C.;     -   optionally surface treating the hollow;     -   cold rolling the hollow, wherein the hollow is displaced forward         in the direction of rolling under cam shaped profiled rolls         rotatably mounted in an oscillated roll stand wherein the hollow         is moved forward stepwise under the rolls, wherein the profiles         of the rolls are shaped so that on rotation they bite into the         hollow to forge it down onto the mandrel that is disposed inside         the hollow, simultaneously the said hollow with the mandrel is         rotated to about its longitudinal axis as hollow advances         between the said rolls wherein the mandrel is then moved back to         drag the hollow against the rolls and the process is repeated         for next segment of the hollow.

In one of the embodiments of the said process there is a simultaneous reduction of tube diameter and tube thickness.

In one of the embodiments the heat treating step is carried out using induction furnace wherein the said hollow is passed through the said furnace at 2-10 meter per minute.

The low carbon cold material is selected from steel comprising 0.04 to 0.45% C, 0.41 to 1.7% Mn, 0.01 to 0.25% Si, 0.004 to 0.011% S, 0.007 to 0.019% P, 0.025 to 0.05% Al and optional 0.01 to 0.03% Nb.

The grades such as SAE 1020, SAE 1026, SAE 1541, SAE 1010, SAE 1012, SAE 1018, SAE 1006, SAE 1018, SAE 1527, SAE 1010 (modified), IS 1079 Gr.D, IS 7048 Gr.3, IS 7048 Gr.D, DIN 17100 St.52.3 but not limited to this are covered in the above compositions.

Welding of the rolled strip at the seams is carried out by means of high frequency induction welder that comprises of induction coil and impeder wherein induction coil induces the electrical current in the metal due to magnetic field generated by the induction coil resulting in generation of heat at the edges of strips resulting in fusion of both the edges with each other. The welded tubes are induction annealed uniformly over the length of the tube such that the difference of hardness of the welded area/heat affected zone and parent material is substantially reduced resulting in substantial reduction in stresses.

The cold rolled tubes produced by the said process typically exhibit finer grain size at the weld and uniformity in the cold rolled microstructure.

The cold rolled tube of the present invention typically exhibit:

-   1. Higher GAM (grain average misorientation) and KAM (kernel average     misorientation) indicating higher amounts of plastic work and more     work hardening; -   2. Approximately, ½ the grain size (of the drawn tube) indicating     more severe plastic deformation; -   3. More homogeneous through thickness (i.e. top, middle and bottom)     microstructure developments (i.e. grain size and misorientation)     -   More stored energy of cold work indicated in XRD (X-ray         diffraction) line profiles that showed broader peaks. -   4. Maximum through thickness variation in the grain orientation is     less than or equal to 14%. -   5. Maximum through thickness variation in the grain average     miorientation (GAM) is less than or equal to 8%. -   6. Maximum through thickness variation in Karnel average     misorientation (KAM) is less than or equal to 8%. -   7. Maximum through thickness variation in the grain size is less     than or equal to 14%.

The tube manufactured using the process of present invention exhibit the above mentioned microstructural properties of the cold rolled tube of the present invention results in enhancement of the mechanical properties of the tube. This is attributed to finer grain sizes in the cold rolled tubes of the present invention that leads to more yield strength. Substantially lower through the thickness variation in the microstructure of the cold rolled tube of the present invention results in enhanced fracture/fatigue properties.

In one of the embodiments in the strip forming operation, pass to pass distance is substantially reduced resulting in reduction of spring back portion.

In another embodiment the heat treatment is carried out by induction means that comprises of AC power supply, induction coil wherein hollow is disposed in the said coil wherein the power supply sends alternating current through the coil resulting in generation of magnetic field that induces eddy currents in the hollow that results in heating the hollow uniformly;

In another aspect of the invention mandrel is provided with internal passages for lubricant that open at the mandrel and internal tube diameter interface to facilitate lubrication at the said surface.

In another embodiment mandrel is provided with tapered profile.

In one of the embodiments the system provides for forming the strip and further welding the same so as to substantially reduce the springing back tendency of the partially formed strip.

In another embodiment the hollow is surface treated to facilitate lubrication between its inner surface and the mandrel during the process of cold rolling.

In another embodiment a system is provided to control tube feed increment and rotational angle in tandem according to the size of the tube. In another embodiment driving system is provided to couple and coordinate process of rotation of the rolls and tube feeding.

The present invention provides a synergistic combination of induction and/or resistance heat treatment of the hollow and the cold rolling process to obviate multiple passes that are essential in processes involving draw bench processes resulting in remarkable reduction in energy consumption, and at the same time enhancing quality of the resulting tubes with improved dimensional stability, closeness of tolerance, reduced thickness variation, concentricity and substantial reduction in material wastage.

The invention is now illustrated with non-limiting examples.

Example Quantification of Energy Efficiency of the Process

Experiment was conducted wherein energy consumed in the novel process was compared with the conventional drawing process of tube manufacture. FIG. 1 and FIG. 2 depict the respective process flow charts. The tubes were manufactured using process of the present invention and the conventional drawing process from the same raw material stock. Energy consumption was measured every stage of the conventional tube drawing process. The total energy consumed in the process of the present invention was measured. It must be noted that the process of the present invention is a single pass process. The specific energy consumption: in kWh/kg was calculated from both the processes to arrive at the net energy saving from the process of the present invention as compared to that consumed in the conventional tube drawing process.

Following are the details of the experiment:

-   -   The experiments were carried out to manufacture tube of outer         diameter 28.58 mm and thickness 3.1 mm (size 28.58 mm×3.1 mm)         using drawing process and cold rolling process of the present         invention     -   Strip cutting and strip forming was carried out as common         process     -   Further, the formed strips were seam welded so as to get the         hollow of 50.80 mm outer diameter and 4.5 mm thickness (size         50.80 mm×4.5 mm)     -   Strip cutting, strip forming and seam welding process to produce         hollow are the common processes for cold drawing as well as cold         rolling process of the present invention     -   Since the rationalizing basis for the energy consumption         comparison is weight of the tubes (and not number of tubes), two         batches (one for cold drawing and one for cold rolling process         of present invention) of 1.8 ton each were processed separately         after production of hollow (refer FIG. 1 and FIG. 2)     -   Following are the details of the drawing process:         -   In the drawing process, to achieve reduction from size 50.80             mm×4.5 mm to size 36 mm×3.8 mm, it is necessary to draw the             tube three times (this is because there is limitation on             reduction achieved in one pass)         -   In the first pass, the hollow is reduced to 44.45 mm×4 mm         -   In the second pass the tube is further reduced to 36×3.6 mm             size         -   Finally in the third pass the tube is reduced to 28.58             mm×3.1 mm size         -   In the first pass, notching process is carried out to             provide holding and gripping end of the hollow while pulling             the same in a drawing process         -   diameter of the hollow is reduced partially in the drawing             process;         -   the tube is then straightened in a tube straightening             system;         -   the drawn tube was then heat treated in the induction             furnace at temperature of 950° C.;         -   The tubes were then processed for surface treatment;         -   This competes first pass wherein the total energy             consumption in the said processes of the first pass was             measured to be 936.9 kWh         -   To reduce size of the tube further second pass was used             wherein the processes mentioned in first were repeated;         -   The heat treatment in the second pass was carried out at             800° C.;         -   The energy consumption in the second pass was measured to be             470.95 kWh;         -   To reduce size of the tube further, third pass was with             repetation of the processes as mentioned above         -   The tube was heat treated at 950° C.         -   The energy consumption in the third pass was measured to be             657.82 kWh     -   Energy consumption was measured in the cold rolling process of         the present invention, following are the details:         -   Tubes were heat treated to the temperature of 950° C. after             seam welding process         -   The velocity of the tube in the induction heating process             was 6 meter per minute         -   Further, the tube was cold rolled in the pilgering process             to the final size of size 36 mm×3.8 mm         -   The total energy consumption measured to be 100.2 kWh

The energy consumption was normalized with respect to the weight of the tubes processed to get the specific energy consumption. It is clear that the cold rolling process of the present invention consumes 0.22 kWh/kg for final tube manufactured as compared to 1.26 kWh/kg for the conventional cold draw process, demonstrating that the process of the present invention consumes 82.1% less energy as compared to that in the conventional drawing process.

Microstructure Analysis was carried out for the tubes manufactured using conventional cold drawing process and cold rolling process of the present invention.

Top, middle and bottom IPF as well as phase and image quality maps were taken wherein scan was taken using FEG EBSD (Electron backscattered diffraction) for tubes manufactured using both the processes. The results are shown in FIGS. 3 to 5.

-   -   The grain size of the pilgered samples of the present invention         is approximately half the grain size of the tube drawn samples         indicating more sever plastic deformation.     -   Pilgered samples show more homogeneous through thickness (i.e.         top, middle and bottom) microstructure developments (i.e. grain         size and misorientation) than the tube drawn material.

Thus finer grain sizes in the pilgered sample of the present invention leads to more yield strength. Apparent uniformity in the pilgered microstructure (through thickness—i.e. between different sections) than the tube drawn material leads to better fracture/fatigue properties. 

1. A cold rolled low carbon welded tube manufacture process comprises steps of: strip cutting as per the desired final tube size; forming of a strip; subjecting the strip to high frequency induction welding along the seam of the said strip to produce hollow wherein the coil in the proximity of the tube and welding rolls induces a magnetic field that is concentrated on the open seam by an impeder which is disposed inside the tube resulting in the generation of heat along the seam to fusion temperature; followed by fusing of the open seam to complete the welding process to produce hollow; heat treating the hollow; optionally surface treating the hollow; cold rolling the hollow, wherein the hollow is displaced forward in the direction of rolling under cam shaped profiled rolls rotatably mounted in an oscillated roll stand wherein the hollow is moved forward stepwise under the rolls, wherein the profiles of the rolls are shaped so that on rotation they bite into the hollow to forge it down onto the mandrel that is disposed inside the hollow, simultaneously the said hollow with the mandrel is rotated about its longitudinal axis as hollow advances between the said rolls wherein the mandrel is then moved back to drag the hollow against the rolls and the process is repeated for next segment of the hollow.
 2. A cold rolled low carbon welded tube manufacture process as claimed in claim 1 wherein the said hollow is heated in the temperature range of 650 to 950° C.
 3. A cold rolled low carbon welded tube manufacture process as claimed in claim 1 wherein the strip forming operation, pass to pass distance is substantially reduced resulting in reduction of spring back portion.
 4. A cold rolled low carbon welded tube manufacture process as claimed in claim 1 wherein formed strip is further welding to substantially reduce the springing back tendency of the partially formed strip.
 5. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein the heat treatment is carried out by induction means that comprises of AC power supply, induction coil wherein hollow is disposed in the said coil wherein the power supply sends alternating current through the coil resulting in generation of magnetic field that induces eddy currents in the hollow that results in heating the hollow uniformly.
 6. A cold rolled low carbon welded tube manufacture process as claimed in claim 1 wherein welding of the rolled strip at the seams is carried out by means of high frequency induction welder that comprises of induction coil and impeder wherein induction coil induces the electrical current in the metal due to magnetic field generated by the induction coil resulting in generation of heat at the edges of strips resulting in fusion of both the edges with each other.
 7. A cold rolled low carbon welded tube manufacture process as claimed in claim 1 wherein heat treatment is a phase transformation annealing resulting in misorientation free and refined grains of the said hollow.
 8. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein heat treatment is carried out in induction furnace wherein hollow is passed through the furnace with velocities in the range of 2 to 10 meter per minute.
 9. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein heat treatment of the hollow is carried out using resistance heating.
 10. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein tube is optionally heat treated after cold rolling.
 11. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein the tube is manufactured from the hollow in a single pass.
 12. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein the low carbon cold material is selected from steel comprising 0.04 to 0.45% C, 0.41 to 1.7% Mn, 0.01 to 0.25% Si, 0.004 to 0.011% S, 0.007 to 0.019% P, 0.025 to 0.05% Al optional 0.01 to 0.03% Nb.
 13. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein the steel is selected from SAE 1020, SAE 1026, SAE 1541, SAE 1010, SAE 1012, SAE 1018, SAE 1006, SAE 1018, SAE 1527, SAE 1010 (modified), IS 1079 Gr.D, IS 7048 Gr.3, IS 7048 Gr.D, DIN 17100 St.52.3.
 14. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein mandrel is provided with internal passages for lubricant that open at the mandrel and internal tube diameter interface to facilitate lubrication at the said surface.
 15. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein hollow is surface treated to facilitate lubrication between its inner surface and mandrel during the process of cold rolling.
 16. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein mandrel is provided with tapered profile.
 17. A cold rolled low carbon welded tube manufacturing process as claimed in claim 1 wherein the control tube feed increment and rotational angle are in tandem according to the size of the tube. 